1. Field of the Invention
The present invention relates to a process for producing a glass shaped material such as a preform having high mass accuracy for precision press-molding directly from a molten glass, and a process for producing an optical element by precision-press-molding a preform prepared by the above process.
2. Related Art Statement
As a method of shaping a preform to which precision press-molding is applied, conventionally, there is employed, for example, (1) a method in which a forward end of a molten glass flow that is flowing out of a flow pipe is received with a shaping mold, the shaping mold is moved downwardly to separate a glass having a predetermined mass without a cutting blade and the glass is shaped into a preform to which the precision press-molding is to be applied (JP-A-2-34525), or (2) a method in which a molten glass that is flowing out of a flow pipe is received with a shaping mold and a received glass is shaped into a preform for precision press-molding while the glass is allowed to float above the shaping mold by applying gas pressure (JP-B-7-51446). These methods are remarkably excellent as a method for producing high-quality preforms.
In conventional precision press-molding, generally, an optical-function surface such as a lens surface is molded (formed) by precision press-molding and a non-optical-function surface is formed by grinding and polishing. In recent years, there are increasing demands for a technique that can be applied to the molding (forming) of the non-optical-function surface of an end product by precision press-molding as well. When the above demands are to be accepted, it is required to bring the volume of a preform into precise agreement with the volume of an optical element to be produced by precision press-molding, i.e., to improve the mass accuracy of the preform. For the above reason, and the like, preforms for precision press-molding are required to have high mass accuracy.
Meanwhile, in the above method (1), a plurality of shaping molds are provided and consecutively carried into a position below a flow pipe and molten glasses are consecutively received and shaped. In this case, it is required to keep the flow of a molten glass at a constant rate and precisely control the timing of separating a molten glass gob equivalent to the mass of one preform from the molten glass flow. For this purpose, it is required to operate every shaping mold in a like manner during the operation from receipt of a forward end of a molten glass to completion of the separation of a molten glass gob. Specifically, it is required to control a change in position and a change in height so that these changes are at very low levels over the time period of operation with regard to each of many shaping molds, and it is hence very difficult to adjust an apparatus.
In the above method (2) and when shaping is carried out by providing a plurality of shaping molds, carrying them consecutively into a position below a flow pipe and allowing the shaping molds, one after another, to receive molten glass, gas that is ejected upwardly for floating a glass in a shaping mold is blown to the flow pipe. Particularly, when a molten glass drop drops down toward the center of a shaping mold, a gas ejection port comes to be positioned right below the flow pipe, so that the gas directly hits the flow pipe. In this state, the temperature of the flow pipe varies, or the behavior of a molten glass drop to drop downward is destabilized, so that there is caused a problem that it is difficult to obtain a glass shaped material having high mass accuracy.